1. Field of the Invention
The invention relates in general to detection and measurement of corona discharges in electrical devices and, more particularly, to quantitative measurement of high frequency corona discharges in flyback transformers for high voltage supplies.
2. Description of the Prior Art
Electrical components operating at high voltages, such as flyback transformers and power supply filter capacitors, isolation transformers, and dielectric materials are usually subjected to a high voltage test prior to use to detect any insulation or structural defects which may lead to corona discharge at operating voltages and frequencies. Intense corona activity within the device will lead to early failure.
A corona discharge has been defined as a type localized discharge resulting from transient gaseous ionization in an insulation system when the voltage stress exceeds a critical value. The ionized gases which make up the corona discharge are highly conductive and may lead to a direct arc discharge across the device. High voltage transformers,including flyback transformers for cathode ray tube power supplies, are particularly susceptible to irregularities in construction, because of the high voltage differential between layers of the windings, and a single misplaced turn or void in encapsulation may result in catastrophic failure from corona discharge. Similarly, capacitors may suffer break down of the dielectric due to corona discharge within the capacitor.
In order to obtain a valid test under conditions which approximate the use of the electrical device in operation, it is desirable to energize the device with high voltage, preferably at a magnitude greater than and a frequency nominally close to that encountered when installed in the intended circuit. Since flyback transformers are energized by a high frequency alternating current (ac) and filter capacitors are also used in such applications, the prior art has attempted to differentiate between the excitation signal and the corona discharge signal by narrow band filters, broad band filters, and cancellation techniques.
One known method for determining the presence of corona discharge in VLF capacitors makes use of corona current signals in the 20 MHz to 50 MHz frequency range to provide a dc level proportional to corona activity. However, it has been found that significant corona activity may occur with devices above the 50 MHz range, which would not be detected by the aforesaid system. Further, no provision is made for testing devices other than in a transmitter circuit operating at VLF. This precludes varying the applied excitation voltage in amplitude or frequency to determine a corona threshold, nor does it allow for testing inductive devices.
Another apparatus for detection of corona discharge signals mixed with noise which is in synchronism with a cycle of a power supply, such as occurs in a flyback transformer of a television receiver, converts the noise and corona signal to a digital signal and subtracts one from the other to provide a resulting digital signal which is converted to an analog signal for observation. This apparatus requires a complex circuit for signal processing to assure phase cancellation of the driving signal.
One difficulty in common with prior art systems is their susceptibility to external electromagnetic radiation sources, which may require the use of a shielded room to preclude errors introduced from such signals. Further, high voltage testing itself produces high levels of radio-frequency interference, which is inimicable to operation in a laboratory or test environment.
The present invention utilizes a modular shielded construction which obviates the need for a shielded room. It provides an alternating current power source variable in frequency and amplitude so that the threshold of frequency dependent corona breakdown in a unit under test can be determined. An absolute power measurement is made by integrating the corona discharge signal over an extended frequency band, so that critical high frequency corona components are included in the measurement. Off-the-shelf devices comprise a substantial portion of the test apparatus, minimizing the need for fabrication of complex, special purpose circuits.